Battery charging circuit generating an oscillating triangular waveform to remove sulphate from battery plates

ABSTRACT

A circuit charges and maintains a battery. The circuit includes a microprocessor for receiving a current sense signal and a voltage sense signal. The microprocessor generates control signals for selectively operating the circuit in a charging mode and in a maintenance mode. The circuit includes a current sense circuit for measuring the battery&#39;s current consumption and generating the current sense signal responsive to the measured current consumption. The circuit includes a voltage sense circuit for measuring the battery&#39;s voltage and generating the voltage sense signal responsive to the measured battery voltage. The circuit includes a waveform generator circuit for receiving the control signals, and operating in response thereto to apply a charging signal to the battery when in the charging mode and to deactivate the charging signal when in the maintenance mode. The charging signal has an oscillating triangular waveform superimposed on a DC bias signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

Pursuant to 35 U.S.C. §119 (e), this application claims priority from,and hereby incorporates by reference for all purposes, U.S. ProvisionalPatent Application Ser. No. 60/975,714, entitled PULSE CHARGING ANDREJUVENATING CIRCUIT, and filed Sep. 27, 2007, U.S. Provisional PatentApplication Ser. No. 60/975,707, entitled METHOD FOR PULSE CHARGING ANDREJUVENATING A BATTERY, and filed Sep. 27, 2007, and U.S. ProvisionalPatent Application Ser. No. 60/975,701, entitled BATTERY PULSE CHARGINGAND REJUVENATING CIRCUIT, and filed Sep. 27, 2007.

FIELD OF THE INVENTION

The invention relates to electronic circuits, and more particularly theinvention relates to a battery charging circuit.

BACKGROUND OF THE INVENTION

A variety of charging circuits are available for charging andmaintaining rechargeable batteries. Existing charging circuits typicallyapply a charging signal to a battery to replenish its charge. Examplesof prior art circuitry used for generating such a charging signal may befound in the U.S. Pat. Nos. 4,871,959, 5,084,664, 5,063,341, 5,276,393,and 5,633,575 and U.S. Reissue Pat. No. RE35,643, the disclosures ofwhich are hereby incorporated by reference.

For increased battery life and performance, rechargeable batteries mustbe protected from excessive charging voltage during recharging. Thus,the battery voltage is monitored during recharging to prevent overcharging. It is known that sulphates build-up on battery plates. Thesesulphate deposits can damage the battery's ability to hold charge and/orbe recharged. At present, charging circuits do not provide an efficientmeans of removal of sulphates during recharging.

SUMMARY

A circuit charges and maintains a battery. The circuit includes amicroprocessor for receiving a current sense signal and a voltage sensesignal. The microprocessor generates control signals for selectivelyoperating the circuit in a charging mode and in a maintenance mode. Thecircuit includes a current sense circuit for measuring the battery'scurrent consumption and generating the current sense signal responsiveto the measured current consumption. The circuit includes a voltagesense circuit for measuring the battery's voltage and generating thevoltage sense signal responsive to the measured battery voltage. Thecircuit includes a waveform generator circuit for receiving the controlsignals, and operating in response thereto to apply a charging signal tothe battery when in the charging mode and to inhibit the charging signalwhen in the maintenance mode.

The charging signal has an oscillating triangular waveform superimposedon a DC bias signal. The oscillating triangular waveform ischaracterized by a relatively short linear rise time followed by asubstantially long linear fall time. The microprocessor operates thecircuit in the charging mode if the measured battery voltage is lessthan a first threshold voltage. The microprocessor operates the circuitin the maintenance mode if the measured voltage exceeds the firstthreshold voltage and the measured current consumption is less than afirst threshold current. During the maintenance mode, the microprocessorapplies the oscillating triangular waveform superimposed on the DC biassignal if the measured voltage is less than a second threshold voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the features, example embodimentsand possible advantages of the present invention, reference is now madeto the detailed description of the invention along with the accompanyingfigures and in which:

FIG. 1 is a block diagram of a battery charger in accordance with anexample embodiment;

FIG. 2 illustrates a pulse charging circuit in accordance with oneembodiment;

FIG. 3A illustrates a waveform generated by the pulse charging circuitin accordance with one embodiment;

FIG. 3B is a waveform generated by a prior art circuit; and

FIG. 4 is a flow diagram of the steps for charging a battery inaccordance with one embodiment.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a block diagram of a battery charger 100 in accordance with anexample embodiment. The battery charger 100 is adapted to receive an ACvoltage (e.g., 85-265V AC) from an AC source such as a wall outlet 110.AC voltage from the wall outlet 110 is filtered at a filtering section114 to reduce line emission. The filtering section 114 may include asafety fuse for over current protection.

A high voltage rectification and step down section 118 is coupled to thefiltering section 114. The high voltage rectification and step downsection 118 provides full-wave rectification of the AC voltage andproduces a pulsating DC voltage. For example, the high voltagerectification and step down section 118 may rectify the 85-265V AC to a120-375V pulsating DC. As will be appreciated, the full-waverectification of a sinusoidal wave produces a pulsating DC voltage.Capacitors are used to filter or smooth out the DC voltage.

In one implementation, the high voltage rectification and step downsection 118 converts the 120-375V DC into a regulated low voltage (e.g.,15V regulated DC). It will be appreciated that the 15 Volts DC issuitable for driving charging current into a typical 12V lead acidbattery. In one implementation, a switching circuit and a step downtransformer is used to convert the 120-375V DC voltage into the 15Vregulated DC.

A microprocessor regulator section 122 and a microprocessor controllersection 134 cooperatively generate a pulse control signal. It will beunderstood that the microprocessor regulator section 122 and themicroprocessor controller section 134 may be incorporated, or merged, ina single device or they may be implemented as separate devices. Themicroprocessor regulator section 122 receives the 15V regulated DC andgenerates a 3.3V regulated DC to power and operate the microprocessor(as well as perhaps, other circuitry). The microprocessor controllersection 134 monitors voltage and current of an external battery beingcharged through sensing circuits, 126 and 130, and responsive to themonitored voltage and current generates the pulse control signal 137. Inone implementation, the microprocessor controller section 134 receives acurrent sense signal 135 from a charge current sense unit 126 andreceives a voltage sense signal 136 from a voltage sense unit 130. Inone implementation, the voltage sense signal 136 may represent acomposite voltage, which may be computed from the battery voltage andthe charging terminal output voltage. In response to the current sensesignal 135 and the voltage sense signal 136, the microprocessorcontroller section 134 generates the pulse control signal 137. A pulsecharging section 138 receives the pulse control signals 137 and inresponse generates a battery charging waveform, which is applied to apositive and a negative terminal, 142 and 146, of the external battery.

In one implementation, the pulse charging section 138 may be implementedwith a pulse charging circuit comprising a pulse transformer. FIG. 2illustrates a pulse charging circuit 200 having a pulse transformer 234.The pulse transformer 234 has a primary winding 235 and a secondarywinding 236. The primary winding 235 has three terminals: terminal #1,terminal #4, and terminal #3 (center tap). A pulse source 214 (e.g.,regulated 15V DC) is coupled to the center tap (terminal #3) of theprimary winding 234 through a limiting resistor 218. A transistor 246includes a collector junction 247 coupled to the terminal #4 of theprimary winding and includes an emitter junction 248 coupled to ground,thus providing a current flow path from the terminal #4 to the groundwhen the transistor 246 is turned ON. A resistor network formed byresistors 238 and 242, coupled between the terminal #1 of the primarywinding and the ground, provide another current flow path from theterminal #1 of the primary winding to the ground. The resistors 238 and242 are coupled to a base junction 249 of the transistor 246. When acurrent flows from the terminal #1 and through the resistors 238 and242, a base current is supplied to the base junction 249 to turn ON thetransistor 246.

As will be appreciated, when the transistor 246 is turned ON, currentflows from the center tap (terminal #3) through the terminal #4 and intothe ground. When the transistor 246 is turned OFF, current flows fromthe center tap (terminal 3) through the terminal 1, the resistors 238and 242 and into the ground.

As discussed before, when current flows from the terminal #1 through theresistors 238 and 242, the transistor 246 is turned ON, causing thetransistor 246 to allow current to flow from the terminal #4 through thetransistor 246 and into the ground. Due to the conduction of currentthrough the transistor 246, current does not flow through the resistors238 and 242, thus removing the necessary base current from the basejunction 249. Consequently, the transistor 246 turns OFF, which in turncauses the current to flow again through the resistors 238 and 242.Thus, the resistors 238 and 242 and the transistor 246 induceoscillation at the primary winding 235 of the pulse transformer 234.

In one implementation, a pulse control signal 222 (which may be thepulse control signal 137 shown in FIG. 1) generated by themicroprocessor (shown in FIG. 1) is utilized to turn OFF the oscillationin the primary winding 235. Specifically, the pulse control signal 222is coupled to a gate junction 231 of a transistor 230 to turn ON thetransistor 230, which in turn pulls the base junction 249 of thetransistor 246 to ground. A resistor 226 couples a 3.3V source to thegate junction 231. When the transistor 230 is turned ON, the transistor246 turns OFF, thus halting the oscillation.

The oscillation in the primary winding 254 of the transformer 234induces AC voltage at terminals 6 and 7 of the secondary winding 236. Atransistor 274 in series with a resistor 278 is coupled between theterminals 6 and 7 of the secondary winding. Thus, when the transistor274 is turned on, current flows from the terminal 6 through and thetransistor 274 to the terminal 7. An R-C network formed by resistors 258and 262 and a capacitor 270 is coupled to a base junction 275 of thetransistor 274. Diodes 254 and 266 couple the R-C network to theterminals 6 and 7, respectively. The diode 266 functions as ahalf-bridge rectifier blocking a return path of the AC current. Thediode 254 clips the oscillating signal wave at 15+ volts causing thebase junction 275 of the transistor 274 to slowly conduct to ground viathe R-C network formed by the resistor 262 and the capacitor 270. Adiode 282 in conjunction with the resistor 278 and the transistor 274produces a pulse waveform at an output terminal 290. A DC bias signal isadded to the pulse waveform, and the resulting pulse waveformsuperimposed on the DC bias signal is applied to the external battery(not shown in FIG. 2). The pulse waveform superimposed on the DC biassignal is also referred to as the battery charging signal. The batterycharging signal is characterized as a triangular or sawtooth signalsuperimposed on a DC bias signal. As will be explained further,depending on the battery condition, the circuit 200 is either operatedin a charging mode or in a maintenance mode. In the charging mode, thebattery charging signal (i.e., the triangular or sawtooth signalsuperimposed on the DC bias signal) is applied to the battery. In themaintenance mode, the battery's settling voltage is continuouslymonitored. If the battery's settling voltage falls below a thresholdvalue, the charging signal is turned ON until the battery's settlingvoltage reaches the threshold value. The charging signal is turned OFFor removed when the battery's settling voltage reaches the thresholdvalue.

The R-C network causes a slow, tapered linear pinch off of transistor274's conductance so as to control the pulse shape. The diode 282prevents the external battery from discharging through the circuit 200.A resettable fuse 286 may be connected in series with the diode 282 forshort circuit protection. In one implementation, the pulse controlsignal 137 (shown in FIG. 1) is used to control a switch 202 to removethe DC bias signal from the output terminal 290. A diode 293 connectedin series with the switch 292 prevents the battery from dischargingthrough the switch 292. It will be understood that the DC bias signalmay be selectively removed from the output terminal 292 using a varietyof other circuitry.

FIG. 3A illustrates a waveform 300 generated by the pulse chargingcircuit 200. As discussed above, the slow, tapered linear pinch off ofthe transistor 274's conduction results in a triangular or sawtoothwaveform. The triangular or sawtooth waveform 300 is superimposed on aDC bias signal 308, and the resulting battery charging waveform is atriangular or sawtooth waveform superimposed on a DC bias signal. Thebattery charging waveform is also referred to herein as a pulsemodulated waveform or a pulse modulated signal. The triangular orsawtooth waveform 300 is characterized by a relatively short linear risetime 302 (e.g., 412 million V/sec) followed by a substantially longlinear fall time 304 (e.g., 40 million V/sec). It will be understoodthat the pulse charging circuit 200 can be modified to generate thetriangular waveform having other rise time and fall time values. In oneimplementation, the ratio of the rise time to fall time is approximately1:10, thus resulting in a relatively short rise time and a substantiallylong fall time. The waveform 300 has a frequency between 30-50 KHz. Theshort linear rise time 302 and a substantially long linear fall time 304of the triangular or sawtooth waveform 300 provides a larger area underthe curve, which enables the waveform 300 to possess increased chargingand desulphating capability. The short linear rise time 302 and therelatively long linear fall time 304 of the triangular or sawtoothwaveform 300 provide increased desulphating voltage and current to theexternal battery being charged. It will be appreciated, the increaseddesulphating voltage and current removes sulphates from the lead platesof the battery, thus improving battery condition.

In one implementation, the pulse charging circuit 200 is operated in acharging mode and a maintenance mode. During the charging mode (when thepulse control signal 222 is active and the DC bias signal is active),the triangular or sawtooth waveform 300 is superimposed on the positiveDC bias signal (e.g., 15V DC) 308. As discussed before, the triangularor sawtooth waveform 300 provides increased desulphating voltage andcurrent to the battery. As will be appreciated, during the chargingmode, the base line of the waveform is the DC bias voltage.

During the maintenance mode, the charging signal (i.e., the triangularor sawtooth waveform superimposed on the DC bias signal) is deactivatedor removed and the battery's settling voltage is continuously monitored.If the battery's settling voltage falls below a threshold value, thecharging signal is turned ON until the battery's settling voltagereaches the threshold value. The charging signal is turned OFF orremoved when the battery's settling voltage exceeds the threshold value.

In one implementation, a microprocessor generates the control signal 137to selectively operate the pulse charging circuit 200 in the chargingmode and the maintenance mode. The charging algorithm may be stored in amemory which is either within the microprocessor or provided as aseparate component connected to the microprocessor. For example, thepulse control signal 222 (also the signals 137) may be generated by themicroprocessor.

FIG. 3B is a waveform 320 generated by a pre-existing circuit. Thewaveform 320 has a frequency of approximately 8 KHZ and exhibits alogarithmic fall time 322, resulting in a comparatively smaller areaunder the curve than the area under the curve of the triangular orsawtooth waveform 300.

FIG. 4 is a flow diagram of the steps for charging a battery inaccordance with one embodiment. The flow starts in step 404 in which anexternal battery is connected to a charger circuit in accordance withthe embodiments discussed above. In step 408, the battery is tested oranalyzed to determine if the battery is defective. If the battery isfound to be defective, the battery is disconnected in step 412 and theflow returns to step 404. If the battery is found not to be defective,the flow moves to step 416 in which the charger circuit is operated in acharging mode.

As discussed before, depending on the charge level of the battery and inresponse to the pulse control signal 222, the charger is operated in acharging mode or in a maintenance mode. During the charging mode, apositive DC charging voltage (e.g., 15V DC) is added to a triangular orsawtooth waveform, and the resulting signal is applied to the battery.The signal is characterized by the triangular or sawtooth waveformsuperimposed on the DC bias signal. The triangular or sawtooth waveformdesulphates the battery by removing sulphate buildup on the lead plates.

In step 420, real time data related to the battery voltage and currentis acquired. In one implementation, voltage and current values arestored in a memory coupled to the microprocessor. In step 424, LEDindicator settings are determined based on the acquired data, and LEDindicators are set to indicate the battery's status. For example, theLED indicators may indicate that the charging circuit is in a chargingmode or in a maintenance mode.

In step 428, a comparison is made whether the battery voltage is lessthan a first threshold voltage (e.g., 14.2V DC). In one implementation,a composite value of the voltage reading of the battery and the chargeris used to estimate the voltage. If the composite value of the voltagereading is less than 14.2V DC, the flow returns to step 420. If thecomposite value of the voltage reading is equal or greater than 14.2VDC, the flow moves to step 432 wherein a determination is made regardingthe current consumption of the battery. The current consumption of thebattery can be determined from the data acquired in step 420. Referringback to step 432, if the current consumption is greater than a firstthreshold current (e.g., 0.75 Amp), the flow returns to step 420. Thus,it will be appreciated that if the battery is drawing more than 0.75Amp, the battery is considered not fully charged by the pulse chargingcircuit (although the battery voltage satisfies the first threshold) andthe flow returns to step 420.

If the current consumption is less than or equal to 0.75 Amp (along withthe high battery voltage), the battery is considered fully charged, andthe flow moves to step 436, wherein the charger circuit is switched tothe maintenance mode. In step 440, the triangular waveform superimposedon the DC bias signal is deactivated or turned OFF. In step 444, thebattery's settling voltage is compared to a second threshold voltage. Ifthe battery's settling voltage falls below a second threshold voltage(e.g., 13.6V DC), the flow moves to step 448 wherein the triangularwaveform superimposed on the DC bias signal is applied to the batteryand the flow subsequently returns to step 444. If the battery's settlingvoltage level remains equal to or above 13.6V DC, the flow returns tostep 440.

While the charging method and operation is described in connection withlead acid batteries, it will be understood that the charging method andoperation disclosed herein may also be used in connection with otherbattery types.

It will also be appreciated that one or more of the elements depicted inthe drawings/figures can also be implemented in a more separated orintegrated manner, or even removed or rendered as inoperable in certaincases, as is useful in accordance with a particular application. It isalso within the spirit and scope of the present invention to implement aprogram or code that can be stored in a machine-readable medium topermit a computer to perform any of the methods described above.

As used in the description herein and throughout the claims that follow,“a”, “an”, and “the” includes plural references unless the contextclearly dictates otherwise. Also, as used in the description herein andthroughout the claims that follow, the meaning of “in” includes “in” and“on” unless the context clearly dictates otherwise.

The foregoing description of illustrated embodiments of the presentinvention, including what is described in the Abstract, is not intendedto be exhaustive or to limit the invention to the precise formsdisclosed herein. While specific embodiments of, and examples for, theinvention are described herein for illustrative purposes only, variousequivalent modifications are possible within the spirit and scope of thepresent invention, as those skilled in the relevant art will recognizeand appreciate. As indicated, these modifications may be made to thepresent invention in light of the foregoing description of illustratedembodiments of the present invention and are to be included within thespirit and scope of the present invention.

Thus, while the present invention has been described herein withreference to particular embodiments thereof, a latitude of modification,various changes and substitutions are intended in the foregoingdisclosures, and it will be appreciated that in some instances somefeatures of embodiments of the invention will be employed without acorresponding use of other features without departing from the scope andspirit of the invention as set forth. Therefore, many modifications maybe made to adapt a particular situation or material to the essentialscope and spirit of the present invention. It is intended that theinvention not be limited to the particular terms used in followingclaims and/or to the particular embodiment disclosed as the best modecontemplated for carrying out this invention, but that the inventionwill include any and all embodiments and equivalents falling within thescope of the appended claims. Thus, the scope of the invention is to bedetermined solely by the appended claims.

1. A circuit for charging a battery, comprising: a microprocessorconfigured to receive a current sense signal and a voltage sense signal,the microprocessor further configured to generate control signals forselectively operating the circuit for charging the battery in a chargingmode and a maintenance mode; a current sense circuit configured tomeasure the battery's current consumption and generate the current sensesignal responsive to the measured current consumption; a voltage sensecircuit configured to measure the battery's voltage and generate thevoltage sense signal responsive to the measured battery voltage; awaveform generator circuit configured to receive the control signals,and operate in response thereto to apply a charging signal to thebattery when in the charging mode and to deactivate the charging signalwhen in the maintenance mode; and a monitoring circuit configured in themaintenance mode to monitor a settling voltage of the battery in theabsence of the charging signal, wherein the charging signal has anoscillating sawtooth current waveform superimposed on a DC bias; andwherein the oscillating sawtooth current waveform is characterized by aplurality of consecutive current pulses, each current pulse having arise portion and a fall portion, wherein the fall portion of eachcurrent pulse comprises a first linear portion having a first slopefollowed by a second linear portion having a second slope different fromthe first slope, and wherein linearity for the second slope of thesecond linear portion is consistent from pulse to pulse in the sawtoothcurrent waveform.
 2. The circuit according to claim 1, wherein themicroprocessor operates the circuit for charging the battery in thecharging mode if the measured battery voltage is less than a firstthreshold voltage.
 3. The circuit according to claim 1, wherein themicroprocessor transfers operation of the circuit for charging thebattery to the maintenance mode if the measured voltage exceeds thefirst threshold voltage and the measured current consumption is lessthan a first threshold current.
 4. The circuit according to claim 3,wherein the microprocessor applies the oscillating sawtooth currentwaveform superimposed on the DC bias if the settling voltage is lessthan a second threshold voltage.
 5. The circuit according to claim 1,wherein the microprocessor is configured to control the waveformgenerator circuit so as to generate the oscillating sawtooth currentwaveform.
 6. The circuit according to claim 1, wherein the sawtoothcurrent waveform has a frequency between 30 KHz and 50 KHz.
 7. Thecircuit according to claim 1, wherein the waveform generator circuitcomprises: a pulse transformer having primary and secondary windings,the primary winding to which is applied the pulse control signals; atransistor in series with a first resistor coupled between the secondarywinding and a ground; and an R-C network formed by a second and a thirdresistor and a capacitor, the R-C network coupled to a base junction ofthe transistor to output the sawtooth current waveform, the R-C networkcoupled to the secondary winding via a first and a second diode,respectively.
 8. The circuit according to claim 7, further comprising athird diode coupling the battery to the secondary winding, the batterybeing charged by the application of the sawtooth current waveform.
 9. Acircuit for charging a battery, comprising: a microprocessor configuredto receive a current sense signal and a voltage sense signal, themicroprocessor further configured to generate a first control signal anda second control signal; a current sense circuit configured to measurethe battery's current consumption and generate the current sense signalresponsive to the measured current consumption; a voltage sense circuitconfigured to measure the battery's voltage and generate the voltagesense signal responsive to the measured battery voltage; and a waveformgenerator circuit configured to receive the first and second controlsignals and a DC bias, the waveform generator circuit further configuredto generate an output signal for application to the battery when in acharging mode as selected by the first and second control signals, theoutput signal including an oscillating sawtooth current waveformsuperimposed on the DC bias, wherein the oscillating sawtooth currentwaveform is characterized by a plurality of consecutive current pulses,each current pulse having a rise portion and a fall portion, wherein thefall portion of each current pulse comprises a first linear portionhaving a first slope followed by a second linear portion having a secondslope different from the first slope, and wherein linearity for thesecond slope of the second linear portion is consistent from pulse topulse in the sawtooth current waveform, the waveform generator circuitfurther configured to deactivate the oscillating sawtooth currentwaveform superimposed on the DC bias when in a maintenance mode asselected by the first and second control signals.
 10. The circuitaccording to claim 9, wherein the first control signal turns on anoscillator circuit and the second control signal switches in the DC biaswhen in the charging mode if the measured battery voltage is less than afirst threshold voltage.
 11. The circuit according to claim 9, whereinthe first control signal turns OFF an oscillator circuit and switchesout the DC bias if the measured voltage exceeds a first thresholdvoltage and the measured current consumption is less than a firstthreshold current.
 12. The circuit according to claim 11, wherein,during the maintenance mode, the first control signal turns ON theoscillating sawtooth current waveform and the second control signalswitches ON the DC bias if a settling voltage of the battery measuredduring maintenance mode is less than a second threshold voltage.
 13. Thecircuit according to claim 9, wherein the sawtooth current waveform hasa frequency between 30 KHz and 50 KHz.
 14. The circuit according toclaim 9, wherein the waveform generator circuit further comprises: apulse transformer having primary and secondary windings, the primarywinding to which is applied the pulse control signals; a transistor inseries with a first resistor coupled between the secondary winding andthe ground; and an R-C network formed by a second and a third resistorand a capacitor, the R-C network coupled to a base junction of thetransistor to enable the circuit for charging the battery to output thesawtooth current waveform, the R-C network coupled to the secondarywinding via a first and a second diode, respectively.
 15. The circuitaccording to claim 9, further comprising a third diode coupling anexternal battery to the secondary winding, the external battery beingcharged by the application of the sawtooth current waveform.
 16. Acircuit, comprising: a current sense circuit configured to measure acurrent consumption of a battery during charging and generate a currentsense signal; a voltage sense circuit configured to measure a voltage ofthe battery and generate a voltage sense signal; a signal generatorconfigured to apply a signal to the battery, the signal generatorcomprising: a first circuit configured to generate an oscillatingsawtooth current waveform, wherein the oscillating sawtooth currentwaveform is characterized by a plurality of consecutive current pulses,each current pulse having a rise portion and a fall portion, wherein thefall portion of each current pulse comprises a first linear portionhaving a first slope followed by a second linear portion having a secondslope different from the first slope, and wherein linearity for thesecond slope of the second linear portion is consistent from pulse topulse in the sawtooth current waveform; and a second circuit configuredto generate a DC bias; a control circuit responsive to the current sensesignal and the voltage sense signal; the control circuit configured tocontrol the signal generator to apply the signal to the battery with theoscillating sawtooth current waveform superimposed on the DC bias if thevoltage sense signal indicates a voltage less than a first threshold,the control circuit further configured to control the signal generatorto inhibit the oscillating sawtooth current waveform superimposed on theDC bias if the voltage exceeds the first threshold and if the currentsense signal indicates the current consumption is less than a secondthreshold.
 17. A circuit, comprising: a signal generator configured toapply a charging signal to a battery, the signal generator comprising: afirst circuit configured to generate a DC bias; a second circuitconfigured to generate an oscillating sawtooth current waveform; a thirdcircuit configured to superimpose the oscillating sawtooth currentwaveform on the DC bias; wherein the oscillating sawtooth currentwaveform is characterized by a plurality of consecutive current pulsesand wherein each current pulse has a rise portion and a fall portion,wherein the fall portion of each current pulse comprises a first linearportion having a first slope followed by a second linear portion havinga second slope different from the first slope, and wherein linearity forthe second slope of the second linear portion is consistent from pulseto pulse of the consecutive current pulses.
 18. The circuit of claim 17further comprising: a control circuit operating responsive to a sensedbattery current and a sensed battery voltage, the control circuitconfigured to superimpose the oscillating sawtooth current waveform onthe DC bias if the sensed battery voltage is less than a firstthreshold, the control circuit further configured to inhibitsuperimposition of the oscillating sawtooth current waveform on the DCbias if the sensed battery voltage exceeds the first threshold and ifthe sensed current is less than a second threshold.